1. Field of the Invention
The present invention relates generally to an image sensing apparatus, and more particularly, to an image sensing apparatus such as a video camera having an automatic iris function of automatically adjusting exposure in response to a video signal obtained from an image sensor.
2. Description of the Background Art
Conventionally, in an image sensing apparatus such as a video camera, it is important to obtain brightness suitable for an object, i.e., a suitable level of a luminance signal obtained from an image sensor. Such a level of a luminance signal includes the average brightness in a picture, i.e., an absolute level of the luminance signal obtained from the image sensor and the contrast of the picture, i.e., a relative level thereof. The former is adjusted by an exposure adjusting operation such as adjustment of an optical iris of a lens and control of the amplification gain of a video signal, while the latter is adjusted by a gamma (.gamma.) correction mechanism.
More specifically, an image sensing apparatus such as a video camera having a function of automatically adjusting exposure in response to a video signal obtained from an image sensor, i.e, a so-called automatic iris function has been put into practice. Such a conventional automatic iris system mainly includes two types: a system of mechanically adjusting exposure by adjusting an optical iris of a lens and a system of electrically adjusting exposure by adjusting the amplification factor of an amplifier for amplifying a level of a video signal obtained from an image sensor.
However, such a conventional automatic iris system has the following disadvantages: when a high luminance portion such as a light source is included in a picture, for example, the amount of stopping down of the lens is increased so that the gain of the entire picture is decreased, whereby a major object becomes insufficiently bright. Contrary to this, when the background is very dark, the amount of stopping down of the lens is decreased so that the gain of the entire picture is increased, whereby the major object becomes too bright.
An exposure correcting method for eliminating such disadvantages has been proposed in Japanese Patent PG,4 Laying-Open No. 110369/1987. In this exposure correcting method, in view of the fact that a major object is arranged in the center of a picture in many cases, the picture is divided into a central region and a peripheral region, a signal level of a video signal in each of the regions is detected as an evaluating value, and the evaluating value in the central region is weighted with respect to the evaluating value in the peripheral region (in the extreme case, the central region "1" while the peripheral region "0"). The size of the aperture of the diaphragm and the amplification gain of the video signal are optically controlled according to the ratio of both the evaluating values, so that contribution of the central region of the picture to the determination of exposure is increased.
On the other hand, the above described gamma correction will be described in detail. In general, a dynamic range of the present image sensor (approximately 40 dB) is much smaller than a dynamic range of the luminance of an object (approximately 100 dB), and a dynamic range of a display device such as a CRT (Cathode Ray Tube) (approximately 30 to 40 dB) is the same as or smaller than that of the image sensor. Thus, even if the brightness of a picture, i.e, an absolute level of a luminance signal is controlled, a high luminance portion in the picture is saturated and a low luminance portion therein is blackened all over, so that it is difficult to obtain the brightness suitable for the entire picture.
In an ordinary image sensing apparatus, gamma correction of a luminance signal is made in a circuit on the side of a camera such that overall gamma characteristics of an entire system, including all devices from an image sensor to a display device, based on photoelectric conversion characteristics (gamma characteristics) of the image sensor and nonlinear photoelectric conversion characteristics of the display device is always 1.
However, considering a case in which the central region of the picture is taken as a priority area to carry out exposure adjustment, to always obtain exposure most suitable for a major object in the central region of the picture, as described above, if there is a substantial difference in luminance between the central region and the peripheral region of the picture, the following problems occur. More specifically, considering a case in which the central region is relatively substantially brighter than the peripheral region, if this central region is controlled to the most suitable luminance level, substantial underexposure occurs in a non-priority portion of the peripheral region. Contrary to this, considering a case in which the central region is relatively substantially darker than the peripheral region, if this central region is controlled to the most suitable luminance level, substantial overexposure occurs in the non-priority portion of the peripheral region.
Additionally, if and when the luminance level in the priority area is extremely small, the S/N (signal-to-noise) ratio is degraded, so that the luminance level frequently fluctuates due to the effect of noises, whereby exposure control becomes unstable.